The process of reverse osmosis is currently in widespread use for the treatment of water. Its value is derived from the ability of a semipermeable membrane to preferentially reject the passage of most salts, a high percentage of organic contaminants, and nearly all particulate matter. The reverse osmosis process, however, has certain well-known limitations and thus is not used as a standalone water purification treatment.
For example, it is customary, and in many cases imperative, to pretreat the water supply ahead of the reverse osmosis to avoid fouling of membrane surfaces. Such pretreatment in many cases limits the performance of the unit because it can reduce the ability of the membranes to reject contaminants. Moreover, in many raw water supplies, calcium and alkalinity levels are sufficiently high that direct treatment by reverse osmosis would cause precipitation of calcium carbonate on membrane surfaces, reducing productivity. To avoid such precipitation, it is known to pretreat the water by addition of polyelectrolyte addition or by acid addition. In the case of acid addition, the effectiveness of the membrane is reduced, and therefore the treated water salt level is increased as is the cost of further removal of dissolved solids downstream of the reverse osmosis unit. Moreover, if acid is added to reduce alkalinity, the alkalinity is converted to carbon dioxide. Carbon dioxide, however, passes freely through the membrane, thus also increasing the cost of downstream treatment.
Attempts to solve the deficiencies of such prior reverse osmosis systems are also well-known. One such system is described in U.S. Pat. No. 4,574,049 to Pittner. This patent describes a chemically enhanced reverse osmosis water purification system in which an inlet of a second reverse osmosis unit is coupled in series to the product water outlet of a first reverse osmosis unit. Additionally, the product from the first reverse osmosis unit is treated with a chemical treatment agent, such as a sodium hydroxide solution, upstream of the inlet of the second reverse osmosis unit. This dual reverse osmosis system is said to alleviate the problems associated with prior art reverse osmosis techniques.
While the two-pass reverse osmosis system of Pittner provides certain advantages over single pass techniques, the permeate output from the first reverse osmosis unit still includes a high amount of carbon dioxide. Therefore, additional costly downstream processing is still required even in the dual pass system.
It is also well-known in the prior art to use equipment to remove carbon dioxide from water to be purified. Conventionally, carbon dioxide removal equipment reduces the anionic loading on and thus the cost of anion exchangers. Carbon dioxide removal is most efficient at low pH values and therefore is performed after a cation exchanger or a reverse osmosis unit. Typically, such removal is effected using either a forced-draft decarbonator or a vacuum degasifier. Vacuum degasifiers are primarily used where it is desired to remove dissolved oxygen down to as low as 20-50 ppb. Forced draft decarbonators are used where dissolved oxygen in the water is not a concern. While such carbon dioxide removal equipment has been used in combination with reverse osmosis treatment units, such systems still have not provided efficient and cost-effective separation of gaseous impurities from the water.
There remains a need to provide improved water purification systems and methods that overcome these and other problems associated with prior art water treatment techniques.